1. Technical Field
The present invention relates to electrical switches, and more particularly, it concerns microswitches.
The invention may be useful as a limit switch in automated control and protection signalling systems in electric drives of machines and mechanisms.
2. Description of the Prior Art
The precision of operation of automatic lines and also control systems for technological processes is much dependent upon the sensitivity of the microswitches employed, which must provide for the precision of transmitted information. It is very difficult in practice to correct the errors made by microswitches.
To actuate movable contacts in microswitches metal plates are preferably used which respond to temperature or pressure and which are subject only to insignificant bending under tension or compression. Therefore, one of the main characteristics of microswitches is sensitivity which is defined by the distance of travel of an actuating member, i.e. by its differential stroke (the shorter the path required for shifting a movable contact in a microswitch the higher its sensitivity).
Known in the art is a microswitch (see, for example, U.S. Pat. No. 2,729,714, Int. Cl.sup.2 HOIH 13/28) comprising an insulating base 1' (see FIGS. 1,2) fixed contacts 2', 3' secured on this base 1', a movable contact 4' adapted to selectively contact the fixed contacts 2' and 3', and means for shifting the movable contact 4' by the use of an actuating member. The means for shifting the movable contact includes an actuating lever 5' and an intermediate lever 6' connected with one another, a flat spring, and a contact lever 8'. The actuating and the contact lever 5' and 8' are connected by their ends in a single point on the insulating base 1'. The movable contact 4' is mounted on the opposite free end of the contact lever 8' whose middle portion is connected with an end of the flat spring 7'. The other end of the flat spring 7' is connected with an end of the intermediate lever 6' and when in its extreme positions it bears up against a limit stop 9' formed on the insulating base 1'. The other end of the intermediate lever 6' is connected with the end of the actuating lever 5' which is opposite to the end connected to the insulating base 1'.
In the initial position of the microswitch, the preliminarily compressed spring 7' exerts a force P upon the contact lever 8', which is connected with it (see FIG. 1).
The contact pressure is equal to: EQU P.sub.k =P Sin .beta.,
where .beta. is an angle of inclination between the intermediate lever 6' and the spring 7'.
The actuating lever 5' moves under the action of an external force F, and the intermediate lever 6' changes its position with respect to the spring 7'.
As the actuating lever 5' moves to the position of operation (when point A takes position A.sub.1), the spring 7' compresses, the force P increase till the value P.sub.1, and the contact pressure P.sub.k somewhat increases to P.sub.k1 =P.sub.1 Sin .beta. (FIG. 2).
As the point A crosses the line of the unstable position of the spring 7', line I--I, shifting the contacts 4' takes place at the inherent speed of motion.
With further motion of the actuating lever 5', point A reaches the final position A.sub.2 (FIG. 1).
When the external force F is removed from the actuating lever 5', all the levers of the means for shifting the movable contacts 4' tend to return to their initial position.
When the point A of the actuating lever 5' crosses the line II--II, the line of the unstable switched position of the spring 7', i.e. when the point A reaches the position of A.sub.3, the reversal shift of the movable contacts 4' takes place.
As can be seen from the description, the contact pressure in the microswitch does not decrease below the nominal value and even slightly increases with the motion of the actuating lever 5' to the position of actuation.
To provide for shifting the contacts 4' in such microswitch, the surfaces defining the motion of the intermediate lever in the limit stop 9' must be located beyond the area defined by the lines of the position of the contact lever 8' in the initial and in the shifted positions.
To provide the shortest differential stroke of the actuating lever 5' that is possible in such microswitch, the spring 7' must be fixed in the middle portion of the contact lever 8', and the limit stop 9' must be located as close to the movable contacts 4' as possible.
The differential stroke of the actuating lever 5' in the point A is equal to: EQU L.sub.A =H+2.DELTA.h,
where
H=distance between the shifted positions of the movable contact 4' PA1 h=magnitude of the stroke of the actuating lever 5'. PA1 The value of the stroke, .DELTA.h, of the actuating lever 5' is calculated from the relation: ##EQU1## then ##EQU2## where L is a horizontal projection of the distance from the axis passing through the contacts 2', 3', to the point O of connection of the spring 7' with the contact lever 8' PA1 .DELTA.L is a horizontal projection of the displacement of the point A from the point O of the connection of the spring 7' with the contact lever 8' required to insure the instantaneous shift of the contacts 4'.
The differential stroke of the actuating lever 5' in the point of application of the force F is equal to: ##EQU3## where L.sub.1, L.sub.2 are horizontal projections of the length of the actuating lever 5.sup.1 and of its portion from the point of the force F application to the point O.sub.1 of its connection with the insulating base 1'.
Another embodiment of the microswitch disclosed in the same U.S. Pat. No. 2,729,714 also comprises an insulating base 1" (FIG. 3), fixed contacts 2", 3" secured on the insulating base 1", a movable contact 4" adapted to selectively engage the fixed contacts 2", 3" and means for shifting the movable contact 4" by the use of an actuating member, which means includes actuating and intermediate levers 5", 6" connected in series with one another, a flat spring 7", and a contact lever 8", and also a limit stop 9" formed on the insulating base 1" and located beyond the area defined by the lines of the positions of the contact lever 8" in the initial and the shifted positions. However, in this microswitch, the flat spring 7" is connected not with the middle portion of the contact lever 8" as it was in the previous microswitch but with the end of the contact lever 8" on which the movable contact 4" is mounted. The length of the contact lever 8" is equal to L.sub.2 +.DELTA.L, and the differential stroke of the actuating lever 5" at the point A and at the point of application of force F is equal to ##EQU4##
Known in the art is also a microswitch (see U.S. Pat. No. 2,228,523 Int. Cl.sup.2 HOIH 13/28) which like the previous microswitches comprises an insulating base 1"', fixed contacts 2"' and 3"' secured to said base, a movable contact 4"', and means for shifting the movable contact 4"', in which an actuating, an intermediate, a spring and a contact levers 5"', 6"', 7"' and 8"', respectively, are connected in series with one another (see FIG. 4).
A limit stop 9"' is located in this microswitch near the axis, O.sub.1, of rotation of the actuating and the contact levers 5"', 8"'. The distance, H.sub.1, between the surfaces in the limit stop 9"' defining the stroke of the intermediate lever can be short, i.e. shorter than the distance, H, between the contacts 2"', 3"', unlike that in the previous microswitches.
The value of H.sub.1 is governed by the angle .beta. and by the rate of the spring 7"' (i.e. by the value of P), which define the contact pressure since P.sub.k =P.sub.2 Sin .beta..sub.1.
The differential stroke of the actuating lever 5"' at the point A is equal to: EQU L.sub.A =H+2.DELTA.h.sub.1.
The value of .DELTA.h.sub.1 is found from the relation: ##EQU5## Hence ##EQU6## Thus ##EQU7##
The differential stroke of the actuating lever 5"' at the point of application of force F is equal to ##EQU8##
As can be seen from comparison of the forms (2), (3), the differential stroke of the actuating lever 5"' in this microswitch shown in FIG. 4 is somewhat smaller than in the previous one shown in FIG. 3 since H+H.sub.1 .ltoreq.3H. ##EQU9##
In the known microswitches shown in FIGS. 1-4 the differential stroke of the actuating lever at the point A is considerable, which reduces their sensitivity.